Enteroviruses: A Gut-Wrenching Game of Entry, Detection, and Evasion

Abstract

Enteroviruses are a major source of human disease, particularly in neonates and young children where infections can range from acute, self-limited febrile illness to meningitis, endocarditis, hepatitis, and acute flaccid myelitis. The enterovirus genus includes poliovirus, coxsackieviruses, echoviruses, enterovirus 71, and enterovirus D68. Enteroviruses primarily infect by the fecal-oral route and target the gastrointestinal epithelium early during their life cycles. In addition, spread via the respiratory tract is possible and some enteroviruses such as enterovirus D68 are preferentially spread via this route. Once internalized, enteroviruses are detected by intracellular proteins that recognize common viral features and trigger antiviral innate immune signaling. However, co-evolution of enteroviruses with humans has allowed them to develop strategies to evade detection or disrupt signaling. In this review, we will discuss how enteroviruses infect the gastrointestinal tract, the mechanisms by which cells detect enterovirus infections, and the strategies enteroviruses use to escape this detection.

Keywords: enteroviruses; gastrointestinal tract; interferon; pattern recognition receptors.

Figure 1

The gastrointestinal tract defenses. (A) The gastrointestinal tract is composed of numerous cell types that are important for immune activation and barrier surface defenses. The gastrointestinal epithelium is composed of enterocytes, goblet cells, Paneth cells, enteroendocrine cells, tuft cells, and stem cells. In contrast, the lamina propria is composed of immune cells such as dendric cells, T cells, and macrophages. (B) Polarized intestinal epithelial cells have distinct apical and basolateral domains. The apical domain contains microvilli and is closely associated with the actin cytoskeletal network.

Figure 2

Enterovirus life cycle. Enteroviruses enter the cell through receptor-mediated endocytosis (1). Following endocytosis, uncoating of the virion occurs in the endosome and the positive-stranded RNA along with the covalently-linked VPg protein is released into the cytoplasm (2 and 3). Viral RNA is translated by host ribosomes making a single polyprotein that is catalytically cleaved by enterovirus proteases 2A^pro and 3C^pro (4, 5, and 6). After production and accumulation of non-structural proteins, including the viral polymerase, viral RNA is then replicated using the virally-encoded RNA-dependent RNA polymerase to generate a double-stranded RNA (8 and 9). The negative sense RNA serves as the template to make more positive sense RNA. This newly produced RNA can be the template to produce more positive sense RNAs or serve as the genome for progeny viruses (10). Capsid proteins assemble and newly synthesized positive-stranded viral RNA is packaged into virion (11). Finally, new progeny virions are released either by non-lytic release, where virions are released in vesicles (not shown), or are released when the cell undergoes lysis (lytic release) (12).

Figure 3

Enterovirus evasion strategies of PRR-mediated signaling. Enteroviruses target innate immune signaling proteins through cleavage by the viral proteases 2A and 3C. Shown are the targets for the CVB (blue), EV-D68 (green) and EV71 (red) proteases. All three viral proteases target MDA5 and the TLR adaptor protein, TRIF, as mechanisms to halt antiviral innate immune signaling in infected cells.

Figure 4

Primary intestinal models to study enterovirus infection. Intestinal crypts are isolated from the small intestine and plated either in Matrigel or on transwells. Crypts plated in Matrigel form a 3D structure called enteroids. When plated in Matrigel, enteroids have an “inside-out” structure, where the apical domain faces inward, and the basolateral domain is facing outward. When crypts are plated on transwells, they form a monolayer that has apical and basolateral polarity. Images shown are from crypts that were plated in Matrigel (top) or on a transwell (bottom) and stained with DAF (green) and CAR (red), which are involved in CVB attachment, uncoating, and entry.